ChuckBaggett

using System.Threading; // Added
using System.Windows.Forms; // Ensure this is present for MessageBox

namespace TextToSpeechApp
{
    internal static class Program
    {
        /// <summary>
        ///  The main entry point for the application.
        /// </summary>

ChuckBaggett

using PiperSharp.Models; // For VoiceModel
using System; // For StringIsNullOrWhiteSpace, ToLowerInvariant, Split, ArgumentNullException

namespace TextToSpeechApp
{
    public class VoiceViewModel
    {
        public VoiceModel Model { get; }
        public string DisplayName { get; }

ChuckBaggett

using System;
using System.IO;
using System.Linq;
using System.Collections.Generic; // Added
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;
using PiperSharp; // For PiperProvider, PiperDownloader
using PiperSharp.Models; // For VoiceModel, PiperConfiguration, AudioOutputType

ChuckBaggett

namespace TextToSpeechApp
{
    partial class Form1
    {
        private System.ComponentModel.IContainer components = null;
        private System.Windows.Forms.Button btnSelectFile;
        private System.Windows.Forms.TextBox txtEditor;
        private System.Windows.Forms.Button btnSelectFolder;
        private System.Windows.Forms.Label lblSelectedFolder;
        private System.Windows.Forms.ComboBox cmbVoiceSelection;

ChuckBaggett

namespace TextToSpeechApp
{
    partial class Form1
    {
        private System.ComponentModel.IContainer components = null;
        private System.Windows.Forms.Button btnSelectFile;
        private System.Windows.Forms.TextBox txtEditor;
        private System.Windows.Forms.Button btnSelectFolder;
        private System.Windows.Forms.Label lblSelectedFolder;
        private System.Windows.Forms.ComboBox cmbVoiceSelection;

ChuckBaggett

I notice that there is no dessert folder. Seem like their should be. 

Sorry if this not the right place for this comment, but it's the best I could fine.